1. Field of the Invention
The present invention relates to the field of power field effect transistors, and more particularly, to the field of multicellular power field effect transistors.
2. Background of the Invention
As the operating frequency of power supplies, including synchronous rectifiers, has increased, field effect transistors have become more attractive as the switching devices for use therein. However, the high dv/dt stresses to which the switching devices in such power supplies are exposed cause the internal body diode of the FET to conduct under normal power supply operating conditions. This internal body diode is a bipolar diode which has an associated stored charge which is relatively large because of the relatively large volume of the drift region in a power FET. Consequently, the recovery time of this diode after forward conduction becomes a substantial portion of an operating cycle as the operating frequency is increased and the conduction in these diodes limits the maximum switching speed and contributes to power losses within the power supply.
A switching device which eliminates these problems is needed. A Schottky diode connected in parallel with an FET has been proposed as a solution to this problem because, in such a circuit, the body diode never turns on if the Schottky diode has lower turn-on and ON-state voltages than the turn-on voltage of the FET's body diode. A fast reverse recovery results since Schottky diodes have very short reverse recovery times. While this circuit can be effective at relatively low operating frequencies, inductances in the connections between the Schottky diode and the FET can have a significant effect on circuit operation as the frequency of operation is increased with the result that the body diode once again becomes conductive. The two above-identified related patent applications overcome the problem of lead inductances by incorporating a Schottky diode in the cell design of a power device.
We have determined, that the structures disclosed in the two above-identified related applications are not ideal for use in a multicellular power FET because incorporation of a Schottky diode in each cell of the structure substantially increases the cell size thereby decreasing the cell density and increasing the specific on-resistance of the device structure. The specific resistance of a device structure is the ON-resistance for a unit area of the active material. Further, in the cell structure shown in the Ser. No. 221,482 application, there is no simple way to include a heavily doped portion in the base region without further increasing the cell size. A heavily doped portion in the base region is considered desirable because it increases the ruggedness of the device structure during turn-off of inductive loads. There is a need for a high power, high speed switching device having a minimum specific resistance and a fast recovery time from reverse conduction.